*From
Progress Report 218 of The Pennsylvania State University, College of
Agriculture, Agricultural Experiment Station, University, Park, Pa., May 1960.
Research of the 1953-54 season was supported by a Grant from Roses Inc..
**Assistant Professor and Professor of Floriculture respectively. Present
address, Ithaca, New York.

The
practice of reducing night temperatures below those normally maintained as a
means of improving flower growth has long been followed by commercial flower
growers. This practice is based on the theory that on cloudy days the amount of
carbohydrates and other food materials produced in the plants is lower than
average because of reduced photosynthetic activity. Lowering the night
temperatures reduces respiration resulting in a decreased utilization of the
carbohydrates, proteins, etc. which should give an improvement in quality of
growth.

The
primary objective of this study was to determine what effect the reduction of
night temperatures, to a reasonable and economic degree, would have on the rate
of development and flower production of greenhouse roses.

A
second objective was to determine what effect the treatment would have on the
lasting quality of cut roses. The supply of carbohydrates in the flower at the
time it is removed from the plant is considered to have an effect on lasting
quality. If the adjustment of night temperature conserves carbohydrates there
should be greater lasting life of the cut flower.

Many
workers have investigated the effects of light on the growth of roses. Post (7)
showed that high night temperature or low light intensity resulted in a
decrease in reserve food and a reduction in color of flowers. Laurie and
Kiplinger (6) found that on dull winter days a reduction in carbohydrates in
roses may occur because the loss by respiration is greater than the gain by
photosynthesis.

Research
at Cornell (3) and Michigan State (4) showed that plants in outside rows of
greenhouse bench produced more bottom breaks and more flowers than when planted
in other less advantageous locations. The improved growth was attributed to
better light conditions in these areas.

Most
of the research on the keeping quality of horticultural crops has been with
various methods of reducing respiration in storage. By reducing respiration,
the utilization of carbohydrates, proteins, etc. would be lowered which in turn
would prolong the life of the stored product.

One
of the easiest methods of reducing respiration is by lowering the temperature
(5). An increase in temperature causes an increase in respiration; similarly a
reduction in temperature has the opposite effect.

Siegelman
(8) showed that respiration of cut roses stored at 59 degrees F proceeded at a
rate about 3 times as great as those stored at 41 degrees F.

It is
interesting to note that Alban and his co-workers (1) observed the effect of
various cultural practices on the respiration rate of the harvested fruit of
greenhouse tomatoes. Tomatoes were grown in three greenhouses at night
temperatures of 57, 62 and 67 degrees F. Respiration rates for harvested fruit
were lowest for those grown in 57, intermediate for those grown in 62 and
highest for those grown in 67 degree night temperature houses.

These
results suggest that roses grown under lowered night temperature would have a
reduced respiration rate and subsequently the cut flowers would have a longer
lasting life.

Experimental Procedure

1952-53
Season

Graft-budded,
"started eye" rose plants of the varieties, Better Times, Golden
Rapture, Peter's Briarcliff, and Lily White, donated by the Jackson and Perkins
Co., were planted February 13, 1952, in experimental plots each consisting of 8
plants of each variety in 2 rows of 4 plants across a 4 foot bench. Seven
replications of each variety were randomized throughout 3 benches in a 27 foot
section of a greenhouse 32 feet in width. In this section night temperatures
were adjusted according to the daily light intensity.

Seven
replications of similar plots were randomized throughout 2 benches in an
adjacent greenhouse section in which night temperatures were maintained at 60±2
F. degrees regardless of the preceding daily light intensity.

The
roses were planted at a 12 by 12 inch spacing in a silt loam soil of medium
texture and medium nutrient concentration in raised concrete benches. Standard
greenhouse production methods of watering, pinching, tying, fertilizing, and
spraying were followed. Flowers were cut each morning.

Heat
and ventilation were thermostatically controlled to maintain the standard night
temperature (SNT) section 60±2 degrees at night, 70 on cloudy days, and 75
degrees on clear days. Following a day that had been mostly cloudy or
completely overcast, the adjusted night temperature (ANT) section was
maintained at a temperature of 55±2 degrees at night, 70 on cloudy days, and 75
degrees on clear days. An adjustment in temperature was made if the sky had
been cloudy for at least 50 percent of the day. Following a clear day, the
temperature in the ANT section was maintained at the same 60±2 degrees night
temperature as in the SNT section.

Outdoor
weather conditions permitted controlled 55 degree night temperatures from
November 3, 1952, to April 30, 1953, a total of 176 days. Of the 176 days, 84
cloudy days warranted reducing the night temperature in the ANT section to 55
degrees. The remaining 92 days were considered to be clear and the night temperatures
were not reduced.

Three
periods of flower production were considered in this season: October 1, 1952,
to June 1, 1953, the total yearly production; November 5, 1952 to April 28,
1953, the period of temperature control; and December 24, 1952, to April 28,
1953. From December 24 to April 28 is the period when the roses produced had
been subjected to controlled temperatures from shoot initiation through to
cutting of the mature flowers. This is the most important period as far as
actual temperature effect is concerned.

1953-54
Seasona

aThe
studies for this season were financed by a Roses
Incorporated
Research
Assistantship to the senior author.

In
the 1953.54 season, temperature control was not achieved until November 6,
1953. The 3 periods of production were: October 7, 1953, to May 4, 1954;
November 11, 1953 to March 30, 1954; and December 23, 1953 to March 30, 1954.
Of the 140 days when temperatures could be controlled at night, 70 were cloudy
enough to warrant reducing the night temperature to 55 degrees in the ANT
section. The remaining 70 days were clear and the night temperature was held at
60 degrees F. In an attempt to check the effect that adjusted night
temperatures would have on timing of a crop, the varieties Better Times and
Golden Rapture were tagged daily. Dated tags were placed on shoots when they
were 3/4 to 1 inch long, and at the time of cutting the flower, a second date
was placed on the tag. When a new shoot on the same cane had grown to a length
of 3/4 to 1 inch, a third date was noted. The tags were dated for a fourth time
and removed when the subsequent flower was cut. Over 1200 roses were thus
tagged between October 1, 1953 and April 1, 1954.

During
this season 1637 roses were used to determine the effect of the preharvest
temperature treatment on the lasting life of the flowers. The cut flowers were
hardened for 24 hours in a 35±2 degree refrigerator and then placed in a 70±5
degree room. Records were kept of the number of days the flowers lasted.

The
flowers were considered to be dead when at least 50 per cent of the blooms
exhibited petal fall when the stems were struck lightly. At this time Better
Times flowers had developed a slight bluing of the petals. Golden Rapture
showed some drying and crinkling of the margins of the oldest petals.

1954-55
Season

For
the season 1954-55 night temperatures were again adjusted as in the two previous
years. During this season the three periods of production considered were:
September 29, 1954 to June 7, 1955; November 24, 1954 to April 12, 1955; and
January 12, 1955 to April 12, 1955.

Temperature control was maintained from November 20, 1954
until April 13, 1955 a period of 143 days. Of this total, 77 days were cloudy
and allowed adjustment of night temperatures to 55 degrees, and 66 days were
clear.

1955-56
Season

In
this season, the experimental procedure was changed. Instead of reducing night
temperatures following cloudy days, the regular 60 degree night temperature was
maintained in the ANT section. Following clear days, the night temperature in
this section was increased to 65±2 degrees to see if this would speed up growth
and flower production. Night temperatures in the SNT section were maintained at
60±2 degrees.

The
three periods of production considered were October 4, 1955 to February 14,
1956; November 9, 1955 to February 14, 1956; and December 28, 1955 to February
14, 1956 when the experiment was discontinued.

During
this season, temperature control was maintained from November 8, 1955 until
February 14, 1956, a total of 101 days. Of this period, 62 days were cloudy
enough to warrant maintaining the 60 degrees night temperature in the ANT
section. There were 39 clear days following which the night temperature was
increased to 65 degrees in the ANT section.

Results and Discussion

1952-53
Season

The
data presented in table 1 show the total flower production per plot of eight
plants, averages of seven replications of the varieties Better Times and Golden
Rapture. Data on these two varieties only are presented since the response of
the varieties Peter's Briarcliff and Lily White was similar to that of Better
Times. From this table it may be seen that lowering night temperature following
cloudy days resulted in a drastic reduction in production of Better Times. This
amounted to 28% less flowers in the period November 5 to April 28 and
approximately 33% less flowers produced in the period December 24 to April 28.

Although
the variety Golden Rapture produced fewer flowers in the adjusted night
temperature (ANT) section compared to the standard night temperature section
(SNT) for the three periods, the differences were negligible. The total
production of Golden Rapture was not as great as Better Times.

Table
1. Flower production of greenhouse roses per plot as influenced by night temperature
treatments for three periods of production; averages of seven replications,
eight plants per plot; 1932-53.

Treatment

Better
Times

Golden
Rapture

October
1 to June 1

SNT*

155.1

118.6

ANT

130.0

112.3

November
5 to April 28

SNT

100.1

68.8

ANT

71.7

62.0

December
24 to April 28

SNT

70.0

45.4

ANT

48.3

39.3

*SNT
Standard night temperature
ANT Adjusted night temperature

1953-54
Season

During
this season the response of the roses to adjusted night temperatures was
similar to that during 1952-53 Better Times produced more flowers in the SNT
section than the ANT section for the three periods of growth, table 2. The
differences were not as great percentage wise for this season as for the first
season of growth.

Golden
Rapture plants again produced approximately the same number of flowers in both
the SNT and ANT sections for all three periods of growth. The difference of one
or two flowers indicates that reducing night temperatures had no great
detrimental effect on the production of Golden Rapture.

Table
2. Flower production of greenhouse roses per plot as influenced by night
temperature
treatments for three periods of production; averages of seven
replications, eight plants per plot; 1953-54.

Treatment

Better
Times

Golden
Rapture

October
7 to May 4

SNT°

144.0

106.6

ANT

126.4

108.4

November
11 to March 30

SNT

86.6

65.3

ANT

72.4

66.1

December
23 to March 30

SNT

593

44.6

ANT

48.3

42.9

*SNT
Standard night temperature
ANT Adjusted night temperature

Effect
on Flower Development

The
effect of night temperature treatments on the time required for development of
Better Times and Golden Rapture roses from flower to flower is shown in table
3. The average number of days required from flower to flower was consistently
greater in the ANT section than when temperatures were 60º every night. In the
ANT section the development of the Better Times flowers was delayed by four
days for stems tagged in November, four days for stems tagged in December,
seven days for stems tagged in January and two days for stems tagged in
February. No difference in time was noted for stems tagged in March since these
flowers were developing during the period when night temperature could no
longer be adequately controlled.

Table
3. Number of days from flower to flower on same stem of Better Times and
Golden
Rapture roses as influenced by night temperature treatments, 1953-54.

AVERAGE
NUMBER OF DAYS

Month
of
Tagging

Standard
Night
Temperature

Adjusted
Night
Temperature

Better
Times

November

50.5

54.1

December

51.5

55.1

January

48.6

55.7

February

48.7

50.0

March

41.4

41.5

April

39.0

38.9

Golden
Rapture

November

55.4

62.0

December

54.6

62.0

January

54.6

58.9

February

48.6

50.5

March

44.7

45.8

April

40.8

39.9

In
the ANT section with Golden Rapture development was delayed by approximately
six and one-half days for stems tagged in November, four days for stems tagged
in December, four days for stems tagged in January, and two days and one day
for stems tagged in February and March, respectively.

The
difference between averages of the SNT section decreased as the period of
observation approached spring. In April the night temperature could no longer
be adjusted following cloudy weather because of warmer outdoor temperatures.
The average number of days from flower to flower in both sections was almost
equal as would be expected.

Effect
on Keeping Quality of Cut Roses

Individual
comparisons on how pre-harvest treatment, for example, five, four, three, two
or one day of reduced night temperature prior to cutting, affected lasting
quality are presented in the senior author's thesis (2). The data in table 4
show a total of 83 comparisons made when night temperatures prior to cutting
were reduced to 55 degrees in the ANT section.

Expressed as percentages of the total
number, the figures show that for 59 per cent of the comparisons there were no
differences in the number of days the flowers lasted when grown in either
section. For 27.8 per cent of the comparisons made, flowers from the SNT section
lasted longer than did those from the ANT section. For 13.2 per cent of the
comparisons, the flowers grown under the ANT conditions lasted longer than did
those grown under the SNT conditions.

The data in table 4 also show the
comparisons of lasting quality when night temperatures were not reduced in the
ANT section, but maintained at 60 degrees following a clear day. A total of 79
comparisons of lasting quality were made. Expressed as percentages of the total
number, the figures show that for 50.6 per cent of the comparisons there were
no differences in the keeping quality of the flowers produced from either
section. For 27.9 per cent of the comparisons, flowers from the SNT section
kept longer than those from the ANT section. In 21.5 per cent of the
comparisons made, the flowers grown under ANT conditions lasted longer than
those grown under the SNT conditions.

These
comparisons show that there was no consistent effect on lasting quality of the
cut flowers whether the plants were grown under SNT or ANT conditions. Thus it
cannot be concluded from these studies that reducing night temperatures to 55°
after cloudy days will improve the keeping quality of the cut roses.

1954-55
Season:

As
was noted in the two previous seasons, Better Times produced more flowers in
the SNT than the ANT section, table 5. The difference for the period January 12
to April 12 although in favor of the SNT section was of a lesser magnitude than
in either of the two previous seasons.

Table
5. flower production of greenhouse roses per plot as influenced by night
temperature treatments
for three periods of production; averages of seven
replications, eight plants per plot; 1954-55.

Treatment

Better Times

Golden
Rapture

September
29 to June 7

SNT*

190.6

140.0

ANT

174.4

135.4

November
24 to April 12

SNT

81.9

51.0

ANT

64.7

46.4

January
12 to April 12

SNT

53.9

32.9

ANT

49.1

33.6

*SNT
Standard night temperature
ANT Adjusted night temperature

The
variety Golden Rapture was erratic in its response to the temperature treatment.
For the overall period of September to June production was greater in the ANT
section. For the period November to April production was greater in the SNT
section. During the critical period of January to April there was essentially
no difference in production in either the ANT or SNT section. 1955-56 Season:

During
this season, temperatures in the ANT section were raised to 65 degrees
following clear days, whereas in the previous three seasons the night
temperature in this section was reduced to 55 degrees following cloudy days.
This difference in treatments plus the termination of the study in February
would conceivably cause a difference in the results expected. Examination of
the data in table 6 shows a striking difference in the production of Golden
Rapture. In the previous seasons, flower production of Golden Rapture was
consistently below that of Better Times regardless of temperature treatments or
production period. During 1955-56 Golden Rapture had production equal to or
greater than Better Times in either temperature treatment for the three periods
noted. This was unusual, since in previous seasons. Golden Rapture consistently
produced fewer flowers than Better Times.

Table
6. Flower production of greenhouse roses per plot as influenced by night
temperature treatments
for three periods of production; averages of seven
replications, eight plants per plot; 1955-56.

Treatment

Better
Times

Golden
Rapture

October
4 to February 14

SNT*

112.1

112.2

ANT

98.4

104.4

November
9 to February 14

SW

67.3

67.9

ANT

54.7

57.3

December
28 to February 14

SNT

31.7

32.0

ANT

24.7

26.4

*SNT
Standard night temperature
ANT Adjusted night temperature

There
are three possible reasons for this lack of difference in production. The first
is that the experiment was ended approximately two months earlier in 1955-56
than in the three preceding seasons. This meant not as many flowers were cut as
in the previous years. Production following February might have been much less
on the average for Golden Rapture and so would have reduced the average
production figures reported.

A
second possibility is that Better Times had started a natural decline in
productivity and Golden Rapture had not, but the figures in table 7 show that
both Better Times and Golden Rapture had an increase in productivity for the
1955-56 season.

Table
7. Average production per plot per week of two varieties of greenhouse roses;
two night temperature treatments combined for four season's growth; 1952-56.

Season

Variety

Better
Times

Golden
Rapture

1952-53

4.07

3.29

1953-54

4.50

3.58

1954-55

5.07

3.68

1955-56

5.54

5.25

The
third possibility is that for reasons unknown, Golden Rapture produced more
flowers during this season than in the preceding years. No other explanation
can be offered for this occurrence.

Increasing
night temperatures following clear days was not the reason since production in
the section where temperatures were maintained at 60 degrees at night was
greater than in the ANT section.

Summary

The
rose varieties Better Times, Golden Rapture, Peter's Briarcliff, and Lily White
were grown under standard and adjusted night temperature conditions for four
seasons. During the first three seasons, night temperatures were reduced to 55
degrees Fahrenheit following cloudy days. In the fourth season night
temperatures were increased to 65 degrees following clear days. A control
section of the greenhouse was maintained at 60 degrees night temperature
regardless of the previous daily light conditions.

In
the three seasons during the period when night temperatures could be reduced to
55 degrees in the ANT section, approximately half of the days were considered
cloudy enough to warrant a reduction to 55 degrees and the remainder were clear
enough to maintain 60 degrees at night.

Reducing
the night temperature to 55 degrees after cloudy days reduced the flower
production and the rate of plant growth of Better Times, Peter's Briarcliff and
Lily White but had negligible effects on Golden Rapture. The treatment resulted
in no increase in keeping life of the flowers of any of the varieties tested.

Increasing
the night temperature to 65 degrees after clear days did not improve the flower
production over the standard night temperatures, but the results of this season
cannot be considered as conclusive because the production period was short,
running only from October 4 to February 14.

Since
production in the SNT section was so much greater in the first season than
production in the ANT section it might be concluded that subsequent improved production
in the SNT section was an accumulative effect carried over from the preceding
year's growth. This might have occurred had the plants been allowed to continue
growth uninterrupted except for normal harvesting practices; however, the roses
were cut-back to a height of 18 inches in June of each year.

Although
the reduction of night temperatures to 55 degrees was not beneficial it must be
remembered that six cloudy days was the longest consecutive period of cloudy
weather that prevailed during the winter of 1953-54 at University Park,
Pennsylvania. Since the growth process is not a simple start and stop function
dependent on light only, the effects of one day's environment may be carried
over for several days, and one adjustment in night temperature may not be
enough to cause any differences to occur. Nevertheless, there is the
possibility that during a prolonged period of cloudy weather for two or three
or more weeks it may be beneficial to maintain night temperatures cooler than
they are normally held.

Until
further studies are made, and on the basis of these experiments, it appears
desirable that standard night temperatures be maintained during fall, winter
and spring regardless of the previous daily light intensity.